1. Field of the Invention
The present invention relates to a vehicle brake hydraulic pressure control apparatus which utilizes normally open linear solenoid valves as inlet valves.
2. Description of Related Art
In general, there is known a vehicle brake hydraulic pressure control apparatus which is disposed between a master cylinder generating hydraulic pressure according to a pedal effort applied by the driver and wheel brakes which apply brake forces to road wheels for controlling brake forces generated at the wheel brakes. A vehicle brake hydraulic pressure control apparatus like this includes mainly normally open inlet valves which allows transmission of the brake hydraulic pressure from the master cylinder side to the wheel brakes, normally closed outlet valves for allowing hydraulic pressures within the wheel brakes (hereinafter, also referred to as caliper pressures) to escape therefrom and reservoirs for absorbing the brake hydraulic pressures which are allowed to escape therefrom by the outlet valves being opened. In addition, in the vehicle brake hydraulic pressure control apparatus, a brake control by a so-called anti-lock brake system (hereinafter, referred to as an ABS control) is enabled in which, for example, when it is determined that one of the road wheels is likely to lock when braking (a slip ratio has reached a predetermined value or higher), the corresponding inlet valve is closed while the corresponding outlet valve is opened so as to allow the caliper pressure to escape into the reservoir to thereby prevent the lock-up of the road wheel.
As the vehicle brake hydraulic pressure control apparatus that has been described above, a vehicle brake hydraulic pressure control apparatus is conventionally known which utilizes as the inlet valves normally open linear solenoid valves whose valve opening amounts can be changed arbitrarily according to their energization amounts (refer to Japanese Patent Unexamined Publication JP-A-2003-19952).
In this technique, as shown in FIG. 8, when a slip ratio (a ratio of vehicle speed to wheel speed) has reached a predetermined value or higher (at time T1) at one of road wheels, a current resulting in a slightly high current value a is supplied to its inlet valve which is normally open, so that the inlet valve is closed at a breath, while its outlet valve is opened to reduce a caliper pressure at a wheel brake of the road wheel. Then, during this ABS control, when the slip ratio has reduced to a value lower than the predetermined value (when the road wheel has been restored from the lock state to the normal state; time T2), the outlet valve is closed, while the current supplied to the inlet valve is controlled so as to open the inlet valve by a predetermined valve opening amount. Specifically, the energization amount is reduced at a breath from the predetermined current value a which corresponds to the closed state to an initial current value β (at time T2). Thereafter, the energization amount is reduced gradually with a predetermined gradient, whereby the caliper pressure is made to increase substantially at the same time as a pressure increasing control is started (at time T2). In addition, when the slip ratio becomes equal to or higher than the predetermined value again in the course of the energization amount being reduced with the predetermined gradient (at time T3), a current of the current value .alpha. is supplied to the inlet valve so that the inlet valve is closed at a breath, whereupon the pressure increasing control ends.
Incidentally, in the current control that has been described above, when the current value is gradually reduced from the initial current value β, in the event that a target current value is determined in advance, the inclination of the gradient can be changed appropriately in accordance with, for example, a road surface friction coefficient. In addition, it is preferable that the target current value takes a current value at which the pressure increasing control ends, and in general, it is desirable to refer to a current value resulting when the last pressure increasing control of the several pressure increasing controls that had been executed during the ABS control ended.
When the current value resulting when the last pressure increasing control ended is used as the target current value as has been described above, however, since the last current value cannot be referred to in an initial pressure increasing control of the ABS control, in the initial pressure increasing control, for example, it has been necessary to perform a current control with a constant gradient with no target current value determined. In addition, when the current control is performed only with the constant gradient, since it takes time to realize an intended caliper pressure depending on a road surface condition, it has not been possible to increase further the efficiency of the brake control.